The prothrombin time (PT) and activated partial thromboplastin time (aPTT) are the most frequently performed coagulation screening tests. They assess distinct parts of the coagulation cascade and are essential for evaluating bleeding risk, monitoring anticoagulant therapy, and diagnosing coagulation factor deficiencies.
Prothrombin Time (PT) Principle
The PT measures the time for plasma to clot after addition of tissue factor (thromboplastin) and calcium, activating the extrinsic (tissue factor) and common pathways. It is sensitive to deficiencies of factor VII (extrinsic), and factors X, V, prothrombin (II), and fibrinogen (I) in the common pathway. The PT is insensitive to deficiencies of factors VIII, IX, XI, and XII (intrinsic pathway). The normal PT range is approximately 11–14 seconds, varying by reagent and instrument.
International Normalized Ratio (INR)
The INR was developed to standardize PT results across different laboratories and thromboplastin reagents. Because thromboplastin reagents vary in sensitivity (measured by the International Sensitivity Index, ISI), the same PT in seconds can give different clinical interpretations. The INR is calculated as (patient PT / mean normal PT)^(ISI). The therapeutic target for warfarin anticoagulation is typically INR 2.0–3.0 (mechanical heart valves: 2.5–3.5). Self-testing INR monitors are available for home use with proper training and quality assurance.
Activated Partial Thromboplastin Time (aPTT) Principle
The aPTT measures the time for plasma to clot after contact activation (using kaolin, silica, or ellagic acid), addition of phospholipid (partial thromboplastin, lacking tissue factor), and calcium. This activates the intrinsic and common pathways. It is sensitive to deficiencies of factors XII, XI, IX, VIII (intrinsic), and factors X, V, prothrombin (II), and fibrinogen (I) (common). The normal aPTT range is approximately 25–35 seconds, again varying by reagent and instrument. The aPTT is less sensitive to factor IX deficiency than factor VIII deficiency, so a normal aPTT does not completely exclude hemophilia B.
Causes of Prolonged PT
Isolated PT prolongation suggests factor VII deficiency (congenital, liver disease, early vitamin K deficiency, or warfarin effect). Combined PT and aPTT prolongation indicates multiple factor deficiencies: liver disease (impaired synthesis of all vitamin K-dependent factors and factor V), disseminated intravascular coagulation (consumption of factors and fibrinogen), vitamin K deficiency (factors II, VII, IX, X), warfarin therapy, or combined factor deficiencies. Lupus anticoagulant (an antiphospholipid antibody) can also prolong PT with certain sensitive reagents.
Causes of Prolonged aPTT
Isolated aPTT prolongation has a broad differential. Factor VIII deficiency (hemophilia A) presents with prolonged aPTT, normal PT, and bleeding symptoms (hemarthroses, soft tissue hematomas). Factor IX deficiency (hemophilia B) is clinically identical. Factor XI deficiency (hemophilia C) typically causes milder bleeding, often injury-related. von Willebrand disease (low vWF reduces factor VIII protection) prolongs aPTT in severe cases. Lupus anticoagulant prolongs aPTT but is associated with thrombosis, not bleeding — distinguished from factor deficiency by a mixing study that does not correct. Heparin therapy (unfractionated heparin) is monitored by aPTT, with therapeutic ranges calibrated to heparin levels (0.3–0.7 IU/mL by anti-Xa). Contact factor deficiencies (factor XII, prekallikrein, HMWK) cause markedly prolonged aPTT with no bleeding symptoms.
Mixing Studies
When PT or aPTT is prolonged, a 1:1 mixing study with normal pooled plasma is performed. Immediate correction (to within normal range) indicates factor deficiency. Incomplete or absent correction suggests an inhibitor (such as lupus anticoagulant, factor VIII inhibitor). Incubation at 37°C for 1–2 hours may demonstrate time-dependent inhibitors (factor VIII antibodies). The mixing study guides subsequent specific factor assays and inhibitor assays (Bethesda assay for factor VIII inhibitors).
Preanalytical Variables
Sample quality is critical: citrate anticoagulant (3.2% sodium citrate, 9:1 blood-to-citrate ratio), proper venipuncture (clean stick, no heparin contamination), correct fill volume (underfilled tubes over-citrate, overfilled tubes under-citrate), no hemolysis or clotting, centrifugation at 1500–2000 g for 15 minutes for platelet-poor plasma, and testing within 4 hours (or frozen at -20°C for batch testing). High hematocrit (> 55%) requires citrate volume adjustment to maintain the correct plasma-to-citrate ratio. Lipemic or icteric samples may interfere with optical clot detection methods.
Clinical Context
The PT and aPTT must be interpreted in the clinical context. A prolonged aPTT in a patient with bleeding suggests hemophilia or von Willebrand disease; in a patient with thrombosis, suggests lupus anticoagulant. Routine pre-operative coagulation testing is not recommended in asymptomatic patients without bleeding history. Unexpected prolongation requires a systematic workup including mixing studies, specific factor assays, lupus anticoagulant testing, and assessment of fibrinogen and platelet function.
